Automatic dispensing devices that deliver liquids, towels, or other materials are known in the prior art. It is also well known that devices for automatically delivering fluid or the like onto the hands of a user, without requiring manual operation thereof or any physical contact therewith, are known in the prior art. It is well known in the art that such a dispenser may employ an electronic control circuit that monitors a voltage generated by a photo sensor to detect the presence of a user. Early hands-free devices employed a light-emitting diode (LED) in communication with a photodiode. When a user was within the target, the photodiode voltage changed and a signal was sent to supply power to a dispenser motor. However, dispensers that employ this technology fail to compensate for changes in ambient lighting conditions. In addition, these dispensers present power requirement concerns, because detection circuitry requires constant power.
A known hands-free dispenser is shown in U.S. Pat. No. 5,772,291 to Byrd et al. The “hands-free” device in the Byrd patent includes both an array of photovoltaic cells and a photo sensor that detects the amount of light present in the room. The array of photovoltaic cells supplies a reference voltage to the negative input, and the photo sensor supplies a “detection” voltage to the positive input. This allows the dispenser's motor to activate when the photo sensor's voltage becomes greater than the ambient light's reference voltage. Since the array of photovoltaic cells provides power to the control circuitry, the dispenser will only function if there is light present in the room. The device in the Byrd et al. patent requires two separate detection devices that require additional hardware.
Also well known in the art are dispensers that measure capacitance as a means of detection. Conductors are arranged on the unit to provide a capacitance value.
One such device is shown in U.S. Pat. No. 6,903,660 to Hansen et al. This “hands-free” device comprises a differential frequency discriminator used in a signal detection circuit, first and second averaging circuits, and a comparator. When the change in average capacitance is greater than what is allowed by set parameters, the motor is powered and a fluid dispensed.
Another common “hands-free” dispenser known within the art employs pulsed signals similar to what is applied in a television set. One such dispenser is found in U.S. Pat. No. 4,786,005 to Hoffman et al. This dispenser contains circuitry, wherein a phototransistor constantly senses the level of ambient light. When the phototransistor senses a drop in ambient light level, the input of the integrated circuit from the phototransistor goes low. The voltage from the phototransistor is modulated and channeled through a transistor to power an LED at a pulsed infrared light, which is distinct from any stray infrared light which may be received by an infrared sensor receiver. The dispenser utilizes a different phototransistor to sense the pulsed light signal, and then communicates to the circuitry that powers the dispenser motor. The detection of stray infrared light can cause unwanted dispensing events.
In light of the foregoing, it is desirable to provide an improved electronic control system for use in an automatic fluid dispenser of the type generally described above that exhibits low average power drain yet provides significant immunity to background noise.